Biomedical subjects

C A Barnes

Publications and source records attributed to C A Barnes.

At least 19 recordsLinked to original sources

PubMed1992

Region-specific age effects on AMPA sensitivity: electrophysiological evidence for loss of synaptic contacts in hippocampal field CA1.

The effects of aging on the responsiveness of hippocampal neurons to iontophoretic application of L-glutamate and AMPA were studied in vitro. There were no effects of age on neuronal responses to L-glutamate; however, CA1 pyramidal cells of old rats, but not granule cells in the fascia dentata, showed both a smaller reduction in extracellularly-recorded synaptic responses following application of AMPA (presumably mediated by depolarization), and smaller extracellular "DC" fields (measured by subtracting the DC potentials at the dendrite and soma following AMPA application in the dendrites). To examine the cellular bases of this age-related alteration in AMPA sensitivity, two additional electrophysiological approaches were used: (1) measurement of the amplitude ratios of extracellular EPSP and fiber potential components of the Schaffer collateral-CA1 response; (2) measurement of intracellularly recorded unitary EPSPs and quantal analysis of their fluctuations. The interpretations that would be placed on four hypothetical possible outcomes of such experiments are outlined and assessed in relation to the experimental data. The pattern of results obtained in the present experiments supports the following conclusions: In old rats, individual Schaffer collateral synapses do not appear to have altered AMPA receptor properties, as neither the mean size of the unitary synaptic response nor the apparent quantal size differs between age groups; however, the data do support the conclusion that there are fewer synapses per Schaffer collateral branch in old versus young CA1 pyramidal cells.

Aging↗

PubMed1991

Expression of lacZ gene fusions affects downstream transcription in yeast.

Chimeric genes containing Escherichia coli lacZ sequences are often used to characterize gene expression in yeast cells. By Northern analysis, we found that such genes produce multiple transcripts due to inefficient 3'-end formation. The same transcript pattern was found for two related chimeric genes when these genes were cloned separately into the commonly used vector, YIp5, and integrated into the yeast genome at two different locations. Each chimeric gene was composed of promoter and N-terminal coding regions from the yeast SSA1 or SSA2 genes fused in-frame to the lac operon. Transcripts were shown to initiate within the yeast promoter fragment, but transcript size indicated that 3' ends were localized to three different regions: within the lac operon near the 3' end of the lacZ gene; near a terminator region previously identified upstream of the URA3 gene in YIp5; and at the URA3 terminator region. Readthrough transcription of the URA3 promoter from upstream lac sequences decreased the basal activity of the URA3 promoter, although induced URA3 transcription levels were unaffected. This readthrough transcription also resulted in a novel, longer URA3 transcript.

Base Sequence↗

PubMed1991

DNA binding of CPF1 is required for optimal centromere function but not for maintaining methionine prototrophy in yeast.

The centromere and promoter factor 1 (CPF1) binds specifically in vitro and in vivo to an octanucleotide (RTCACRTG). This sequence is found in the centromere DNA element I (CDEI) of yeast centromeres and upstream from a number of transcription units including MET25, GAL2 and TRP1. Inactivation of the CPF1 gene results in three phenotypes; slow growth, a partial loss of centromere function and methionine auxotrophy. These phenotypes correlate well with the known binding sites for CPF1 and have led to the suggestion that CPF1 functions as a kinetochore protein at centromeres and as a transcriptional activator at promoters such as MET25. By analysing transcription from the MET25, GAL2, and TRP1 genes in cpf1 strains, we demonstrate that CPF1 plays no direct role in their transcriptional regulation. Further evidence in support of this comes from the analysis of point mutations in the basic region of CPF1 that affect DNA binding. A strain expressing a non-DNA bound form of CPF1 is phenotypically Met+, shows normal growth rate but has sub-optimal centromere function. We conclude that a DNA-bound form of CPF1 is required for the kinetochore function but not for maintaining methionine prototrophy.

Amino Acid Sequence↗

PubMed1991

The influence of postmortem delay on evoked hippocampal field potentials in the in vitro slice preparation.

The influence of postmortem delay, and thus in situ anoxia/ischemia, on the recovery, viability, and maintenance of in vitro hippocampal slices was studied parametrically. Brains from rats were kept in situ for delays of 5, 30, 60, 90, 120, or 180 min before dissection and slicing of the tissue. Using standard in vitro conditions, Schaffer collateral and perforant path-evoked field potentials were recorded in the respective cell layers at 2, 4, and 8 h after beginning in vitro maintenance. With submaximal stimulus intensities the percentage of slices displaying at least a 1-mV population spike remained above 20% for all postmortem delays, and this measure of viability was indistinguishable (65%) across the delays of 5 and 30 min in both CA1 and fascia dentata (FD). The amplitudes of the EPSP and population spike and the population spike-to-EPSP ratio tended to decline with anoxic delay in both CA1 and FD; however, approximately half-maximal population spikes averaged greater than 3 mV, even for the longest postmortem delay of 3 h. These results indicate that the delay between death and preparation of in vitro hippocampal slices is less important for obtaining physiologically viable slices than previously believed. These data also imply that meaningful electrophysiological information about premortem brain conditions may be inferred from nervous system tissue which is not available immediately after death.

Action Potentials↗

PubMed1991

Specificity of functional changes during normal brain aging.

Although there is no question that age-related alterations in neural function occur in the central nervous system (CNS) of mammals, these changes tend to be somewhat circumscribed rather than completely global, even in pathological conditions. Examples of this kind of specificity of functional neural change are drawn from experiments on the electrophysiology of the senescent rat hippocampus.

Aging↗

PubMed1990

Role of the medial septum and hippocampal theta rhythm in exploration-related synaptic efficacy changes in rat fascia dentata.

Animals transferred from their home cages to a different environment exhibited an increase in exploratory behavior which was accompanied by a substantial increase in perforant path-evoked population excitatory postsynaptic potentials and decreases in both the areas and the onset latencies of population spikes. As reported previously, these changes substantially outlasted the exploratory behaviors that induced them. Electrolytic lesions of the medial septum severely attenuated the theta rhythm of the hippocampal EEG, but had no significant effect on the exploration related changes in the synaptic and postsynaptic components of the evoked response. In urethane-anesthetized animals, long trains of hippocampal theta produced by sensory stimulation failed to affect the amplitude of evoked responses. These results show that the information critical for the exploration-related alterations in dentate evoked responses does not originate in or pass through the medial septum, and that the changes are not linked to hippocampal EEG states.

Animals↗

PubMed1990

Reversible inactivation of the medial septum differentially affects two forms of learning in rats.

The contribution of the medial septum to different aspects of spatial information processing was assessed by examining the effects of reversible septal inactivation on radial maze performance of rats. In addition, the selectivity with which the medial septum affects learning was studied by testing the effects of septal inactivation on the acquisition of non-spatial information. Rats were first trained according to a spatial working memory procedure that included a 30-min delay between the first 4 (forced) choices and subsequent test (free) choices. The forced choices comprised the sample phase of the experiment while the free choices comprised the test phase. Saline or tetracaine (a local anesthetic) was injected into the medial septal area either before the sample phase, after the sample phase (i.e. at the beginning of the delay period), or just before the test phase. In contrast to the saline injections, tetracaine injected just before the sample or test phases produced a significant increase in errors at test. Tetracaine injection at the beginning of the delay period did not affect test choice accuracy. EEG records showed that septal inactivation drastically, yet temporarily, reduced the hippocampal theta rhythm. Thus, when septal inactivation occurred either before the sample phase or at the beginning of the delay period, hippocampal theta recovered by the time of the test phase. Septal inactivation also produced a significant retardation of learning on a non-spatial reference memory task, although clear improvement over trials did occur. Moreover, the results of subsequent saline injections suggest that at least some of the performance deficit was due to variables other than learning per se.(ABSTRACT TRUNCATED AT 250 WORDS)

Analysis of Variance↗

PubMed1990

CPF1, a yeast protein which functions in centromeres and promoters.

Centromeres and several promoters of Saccharomyces cerevisiae contain a highly conserved octanucleotide, RTCACRTG, called CDEI. Using biochemical, genetic and structural analyses, we show that the same protein binds in vivo to CDEI sites in centromeres and in promoters. This protein, called CPF1 for centromere promoter factor, binds DNA as a dimer. Inactivation of the gene is not lethal but leads to a partial loss of the centromere function and to a Met- phenotype. Changes of the chromatin structure due to inactivation of CPF1 are seen at centromeres and at several CDEI-carrying promoters (e.g. MET25, TRP1, GAL2). However promoter activities are affected in diverse ways making it presently difficult to describe a function for CPF1 in gene expression. The sequence of the cloned gene reveals in the carboxy-terminal part two potential amphipathic helices preceded by a positively charged stretch of amino acids very similar to the helix-loop-helix domains recently identified in factors controlling tissue specific transcription in higher eukaryotes. Carboxy-terminal truncations of CPF1 lacking this domain no longer bind to CDEI. The amino-terminal half of CPF1 carries two clusters of negatively charged amino acid residues. Surprisingly, deletions of these clusters still render cells Met+ and lead only to a marginal decrease in centromere activity.

Amino Acid Sequence↗

PubMed1990

Behavioral correlates of theta-on and theta-off cells recorded from hippocampal formation of mature young and aged rats.

Most hippocampal formation single units in freely behaving rats fall into one of two categories (Ranck 1973). The most obvious behavioral correlate of complex-spike (CS) cells is spatially selective discharge (O'Keefe and Dostrovsky 1971), while theta cells show increased firing in phase with the EEG theta rhythm associated with Vanderwolf's Type I behaviors (e.g. walking, exploration). Recently, Colom and Bland (1987) described, in urethane anesthetized animals, a class of non-CS cell which was inactive in the presence of EEG theta and discharged continuously during LIA. They called these "theta-off" cells and used the term "theta-on" to refer to the classical "theta" cell. We describe the behavioral correlates of 14 theta-off cells encountered in CA1 (n = 1), hilus fascia dentata (FD; n = 4), subiculum (n = 6), abd entorhinal cortex (n = 3). These cells were encountered very infrequently in the course of several experimental investigations of mature young and old rats involving 885 hippocampal neurons recorded from 33 rats during radial maze performance. Fourteen theta-on cells encountered within a few hundred microns of the sites where theta-off cells were recorded were included for comparison. Both theta-on and theta-off cells discharged single spikes and did not show CS bursting characteristic of pyramidal cells. Theta-off cells, however, exhibited significantly greater spike durations than theta-on cells. Mean rates for theta-on and theta-off cells were 8.7 Hz and 6.5 Hz, respectively. Maximum rates were 114 Hz and 104 Hz, respectively. Some cells of both types showed 6-8 Hz modulation while animals traversed the maze.(ABSTRACT TRUNCATED AT 250 WORDS)

Aging↗

PubMed1990

Comparison of spatial and temporal characteristics of neuronal activity in sequential stages of hippocampal processing.

The activity of individual pyramidal cells in the CA1 and CA3 subfields of the rodent hippocampus exhibits a remarkable selectivity for specific locations and orientations of the rat within spatially-extended environments. These cells exhibit high rates of activity when the animal is present within restricted regions of space, referred to as place fields, and are extremely quiet when it is elsewhere. Although this phenomenon has been well studied in the CA fields of the hippocampus, relatively little is known about the spatial and temporal firing characteristics either of the entorhinal cortical inputs to the hippocampus, or of the subicular recipients of the output of hippocampal place cells. We report here on a comparison of spatial and temporal discharge characteristics among entorhinal cortex, CA3 and CA1, and the subiculum. CA3 complex spike cells were significantly more spatially specific than their CA1 counterparts. Neither entorhinal cortex nor subiculum exhibited the highly localized patterns of spatial firing observed in the CA fields. In addition, average discharge rates in these areas were substantially higher. However, particularly in subiculum, there was evidence for spatially consistent, but dispersed, firing in some cells, suggestive of the convergence of a number of CA1 place cells. The patterns observed are not consistent with the hypothesis that spatial selectivity is progressively refined at the various levels of hippocampal processing. Rather, hippocampal output appears to be expressed as a much more highly distributed spatial code than activity within the hippocampus proper. We suggest that the sparse coding used within the hippocampus itself represents a mechanism for increasing the storage capacity of a network whose function is to form associations rapidly.

Action Potentials↗

PubMed1990

Effects of aging on the dynamics of information processing and synaptic weight changes in the mammalian hippocampus.

It is clear that the properties of LTE make it a plausible mechanism for associative information storage at some synapses in the central nervous system. While many of the factors that regulate LTE's induction and expression have been discovered and a strong case is being developed for its role in learning and memory processes, until we understand more clearly the mechanisms underlying both the expression and maintenance of LTE, an understanding of its change with age will be difficult. Judging by the progress that has been made over the past several years in uncovering some of the molecular events that are critical for LTE's expression, one may be optimistic that answers will be forthcoming reasonably soon. Of particular importance to aging mammals, such answers may provide insights into why older organisms show faster forgetting. This may have a profound impact on therapeutic strategies for memory disorders in both normal and pathological conditions of aging.

Aging↗

PubMed1990

Genetic assessment of stationary phase for cells of the yeast Saccharomyces cerevisiae.

Starvation of cells of the yeast Saccharomyces cerevisiae causes cessation of proliferation and acquisition of characteristic physiological properties. The stationary-phase state that results represents a unique developmental state, as shown by a novel conditional phenotype (M. A. Drebot, G. C. Johnston, and R. A. Singer, Proc. Natl. Acad. Sci. USA 84:7948-7952, 1987): mutant cells cannot proliferate at the restrictive temperature when stimulated to reenter the mitotic cell cycle from stationary phase but are unaffected and continue proliferation indefinitely if transferred to the restrictive temperature during exponential growth. We have exploited this reentry mutant phenotype to demonstrate that the same stationary-phase state is generated by nitrogen, sulfur, or carbon starvation and by the cdc25-1 mutation, which conditionally impairs the cyclic AMP-mediated signal transduction pathway. We also show that heat shock, a treatment that elicits physiological perturbations associated with stationary phase, does not cause cells to enter stationary phase. The physiological properties associated with stationary phase therefore do not result from residence in stationary phase but from the stress conditions that bring about stationary phase.

Cell Cycle↗

PubMed1990

Thermotolerance is independent of induction of the full spectrum of heat shock proteins and of cell cycle blockage in the yeast Saccharomyces cerevisiae.

Cells of the yeast Saccharomyces cerevisiae are known to acquire thermotolerance in response to the stresses of starvation or heat shock. We show here through the use of cell cycle inhibitors that blockage of yeast cells in the G1, S, or G2 phases of the mitotic cell cycle is not a stress that induces thermotolerance; arrested cells remained as sensitive to thermal killing as proliferating cells. These G1- or S-phase-arrested cells were unimpaired in the acquisition of thermotolerance when subjected to a mild heat shock by incubation at 37 degrees C. One cell cycle inhibitor, o-phenanthroline, did in fact cause cells to become thermotolerant but without induction of the characteristic pattern of heat shock proteins. Thermal induction of heat shock protein synthesis was unaffected; the o-phenanthroline-treated cells could still synthesize heat shock proteins upon transfer to 37 degrees C. Use of a novel mutant conditionally defective only for the resumption of proliferation from stationary phase (M. A. Drebot, G. C. Johnston, and R. A. Singer, Proc. Natl. Acad. Sci. USA 84:7948-7952, 1987) indicated that o-phenanthroline inhibition produces a stationary-phase arrest, a finding which is consistent with the increased thermotolerance and regulated cessation of proliferation exhibited by the inhibited cells. These findings show that the acquired thermotolerance of cells is unrelated to blockage of the mitotic cell cycle or to the rapid synthesis of the characteristic spectrum of heat shock proteins.

Adenosine↗

PubMed1990

Exploration-dependent modulation of evoked responses in fascia dentata: dissociation of motor, EEG, and sensory factors and evidence for a synaptic efficacy change.

Sharp et al. (1986, 1989) described several alterations in hippocampal responses to perforant path stimulation which accompany exploratory activity. Animals transferred from one environment to another exhibited substantial increases in population EPSPs, and large decreases in both the amplitude and the peak latency of population spikes. These changes are unlike the "gating" effects of behavioral state reported by Winson and Abzug (1977, 1978) in that they considerably outlast the behaviors that produce them and (as shown here) can be dissociated from the EEG state of the hippocampus. In this report we describe several new observations that aid in the interpretation of the possible mechanisms and functional significance of these effects. Transportation of animals from their home environment to a different location was accompanied by: (1) an increase in the incidence of exploratory behavior, and EEG theta rhythm, (2) a substantial, reliable, and persistent (more than 15 min) elevation in the size of the evoked field EPSP recorded at the site of synaptic activation in the stratum moleculare and in the stratum granulosum/hilus, and (3) a reduction in the evoked population spike, and (paradoxically) in the latencies to spike onset and peak. There were no associated changes in the spike threshold (defined as the magnitude of the field EPSP at spike onset), the amplitude of the presynaptic fiber volley, or the fractional change in the second synaptic response during paired-pulse stimulation (suggesting the absence of a change in fractional transmitter release). Neither the motor component of exploratory behaviors nor the hippocampal theta rhythm itself was sufficient to account for the changes in the synaptic and spike components of the response. Thus, the changes may depend on reorientation of the animal's sensorium during exploration, rather than on movement per se or the associated hippocampal theta rhythm. The results of these experiments suggest that exploration-related EPSP growth is due neither to granule cell hyperpolarization nor to changes in the distribution of current sources and sinks on granule cells. Rather, the data suggest that exploratory behavior is accompanied by a substantial increase in evoked synaptic efficacy in granule cells, an increase that long outlasts the behavior that produces it. A possible role for these alterations in information processing and memory is discussed.

Adaptation, Physiological↗

PubMed1989

Recovery of spatial learning deficits after decay of electrically induced synaptic enhancement in the hippocampus.

A widespread interest in a long-lasting form of synaptic enhancement in hippocampal circuits has arisen largely because it might reflect the activation of physiological mechanisms that underlie rapid associative learning. As its induction normally requires the 'Hebbian' association of activity on a number of input fibres, we refer to the process as long-term enhancement (LTE) rather than long-term potentiation (LTP), to emphasize its distinction from the ubiquitous, non-associative 'potentiation' phenomena that occur at most synapses, including those exhibiting LTE. Among other evidence that LTE might actually have a role in associative memory is the demonstration that repeated high-frequency stimulation, which saturated the inducible LTE, caused a severe deficit in spatial learning, although it had no effect on well established spatial memory. These results were consistent with a widespread view that information need only temporarily be stored in the hippocampal formation in order for long-term memories to be established in neocortical circuits. In this context, it is important to understand whether the possible underlying synaptic changes are of a permanent character, or are relatively transient. A second question is whether the actual cause of the observed learning deficit is the distruption of the synaptic weight distribution, and/or the limitation of further synaptic change, which presumably results from experimental saturation of the LTE mechanism. Alternatively, the deficit could be a consequence of some unobserved secondary effect of the high-frequency electrical stimulation. Here we demonstrate that learning capacity recovers in about the same time that it takes LTE to decay, which strongly favours the first possibility and supports the idea that LTE-like processes actually underlie associative memory.

Animals↗

PubMed1989

Reversible inactivation of the medial septum: selective effects on the spontaneous unit activity of different hippocampal cell types.

The contribution of septal afferents to spontaneous hippocampal single unit activity was examined by reversibly inactivating the medial septal nucleus using microinjections of the local anesthetic lidocaine. Septal inactivation reduced spontaneous firing of cells in stratum granulosum and in the hilar/CA3 region for periods of up to about 15 min. The firing rates of CA1 complex-spike (pyramidal) cells, however, were not changed, although CA1 theta cells (inhibitory interneurons) exhibited a significant reduction in spontaneous rate. One interpretation of this pattern of results is that the output of CA1 pyramidal cells is maintained roughly constant in spite of reduced input from CA3 because of a proportional reduction in feedforward inhibition. This interpretation is consistent with Marr's 22 formulation of the manner in which the hippocampus implements distributed associative memory. Alternatively, afferents to CA1 originating from regions other than CA3 may play a larger role in regulating CA1 output than previously assumed.

Action Potentials↗

PubMed1989

Hippocampal granule cells are necessary for normal spatial learning but not for spatially-selective pyramidal cell discharge.

The effects of massive destruction of granule cells of the fascia dentata on the spatial and temporal firing characteristics of pyramidal cells in the CA1 and CA3 subfields of the hippocampus were examined in freely moving rats. Microinjections of the neurotoxin colchicine were made at a number of levels along the septo-temporal axis of the dentate gyri of both hemispheres, resulting in destruction of over 75% of the granule cells. By contrast there was relatively little damage to the pyramidal cell fields. As assessed by three different behavioral tests, the colchicine treatment resulted in severe spatial learning deficits. Single units were recorded from the CA1 and CA3 subfields using the stereotrode recording method while the animals performed a forced choice behavioral task on the radial 8-arm maze. Considering the extent of damage to the dentate gyrus, which has hitherto been considered to be the main source of afferent information to the CA fields, there was remarkably little effect on the spatial selectivity of "place cell" discharge on the maze, as compared to recordings from control animals. There was, however, a change in the temporal firing characteristics of these cells, which was manifested primarily as an increase in the likelihood of burst discharge. The main conclusion derived from these findings is that most of the spatial information exhibited by hippocampal pyramidal cells is likely to be transmitted from the cortex by routes other than the traditional "trisynaptic circuit". These routes may include the direct projections from entorhinal layers II and III to CA3 and CA1, respectively.

Animals↗

PubMed1989

A comparison of supramammillary and medial septal influences on hippocampal field potentials and single-unit activity.

1. A comparison was made between the influences of supramammillary (SUM) and medial septal (MS) nuclei on hippocampal physiology in Nembutal-anesthetized rats. Specifically, the effects of prestimulation of the SUM or MS on the perforant path-dentate field potential, on spontaneous activity of single units, and on perforant path-induced unit activation were assessed. Another series of experiments addressed the issue of whether the SUM and MS effects on the perforant path-dentate field response are independent. 2. Prestimulation of the SUM or MS significantly facilitated the perforant path-dentate population spike with no clear effect on the field excitatory postsynaptic potential (EPSP) recorded in the subgranular zone of the dentate hilus. Prestimulation of either nucleus also reduced the threshold for spike onset. The major differences between the two spike facilitation effects were the magnitude of the change and possibly the optimal interstimulus intervals required to obtain the effects. 3. Acute transection of the ipsilateral column of fornix or dorsal fornix eliminated the SUM population spike facilitation effect. MS lesion or dorsal fornix/fimbria transection eliminated the MS spike facilitation effect. The MS lesion did not alter the effects of SUM prestimulation. Cingulum or medial forebrain bundle transection affected neither SUM- nor MS-mediated spike facilitation. Thus the SUM and MS influences on the dentate field response appear to be independent of one another. The relevant SUM afferents travel through the ipsilateral column of fornix and dorsal fornix, whereas MS afferents project through the dorsal fornix/fimbria. 4. Single units recorded in stratum granulosum (SG) were assessed with respect to several parameters. These included the mean firing rate, whether or not excitation occurred prior to the field population spike and at lower threshold, and whether or not a driven unit responded to a second perforant path stimulus delivered at short latency following the first (during the period of population spike depression). The latter parameter in particular appeared to separate SG cells into two classes. The cells that were not activated during the second field potential were classified as granule cells, whereas those that were activated were classified as basket cells. Based on this distinction, significant differences were also found between the two cell classes on the other parameters. In particular, cells classified as granule cells often had very low firing rates.(ABSTRACT TRUNCATED AT 400 WORDS)

Action Potentials↗